Cladding pumped fibre lasers are becoming well-known. A core of an optical fibre is doped with a rare earth element to become an active gain medium and pump radiation is applied via a cladding layer. The pump radiation is normally produced by a plurality of semiconductor laser diodes, and a means is provided for coupling their output into the cladding of the rare earth doped fibre. Bragg gratings formed in the core of the optical fibre may be employed as mirrors to form a laser cavity.
Some pump diodes are sensitive to optical feedback from the laser and can be damaged, particularly if the laser is operating in a pulsed mode. In general, the amount of feedback experienced by the pump diodes depends upon the means of coupling between the pump diodes and the cladding pumped fibre.
The main features of previous cladding-pumped fibre laser designs are a multi-clad optical fibre having a rare-earth-doped core, a pump source which is usually a semiconductor laser diode, a means of coupling the pump radiation into the cladding of the multi-clad fibre, and a means of coupling a laser signal into or out of the core of the fibre. As used in the following summary, detailed description, and appended claims the words “pump and signal multiplexing”, “de-multiplexing” and the like generally refer to methods of coupling pump radiation into or out of a cladding layer of the multi-clad fibre and coupling signal radiation into or out of the core of the multi-clad fibre will henceforth be referred to as “pump and signal multiplexing” and it is to be understood that this term might be applied to a device which is “de-multiplexing” one or a plurality of the beams.
One feature that differentiates previous cladding pumped fibre lasers from each other is the means of pump and signal multiplexing. Some of these can be classified as:
1. Distributed fibre to fibre pump transfer
2. Bulk optic end-coupling
3. Side coupling
4. Tapered fibre bundle end-coupling
U.S. Pat. No. 4,553,238 (Shaw 1985) teaches a distributed fibre to fibre pump transfer method in which pump radiation couples between parallel pump and signal fibres, as does U.S. Pat. No. 6,826,335 (Grudinin 2000). U.S. Pat. No. 4,829,529 (Kafka 1987) teaches bulk optic end-coupling. U.S. Pat. No. 4,815,079 (1989 Polaroid) and U.S. Pat. No. 5,999,673 (1994 Gapontsev) teach side coupling schemes. U.S. Pat. No. 5,864,644 (1997 DiGiovanni) has been the principal proponent of end-coupling with a tapered fibre bundle and there have been subsequent similar teachings by Fidric U.S. Pat. No. 6,434,302, WO2005/029146 A1 Gonthier, and Dong U.S. Pat. No. 7,016,573 amongst others.
Another significant feature of previous laser designs is the multi-clad optical fibre with rare-earth-doped core. These fibres typically have a core doped with a rare earth element such as ytterbium, erbium, neodymium, or thulium and at least two cladding layers surrounding the core having successively decreasing refractive indices. These fibres are referred to as double-clad: see for example U.S. Pat. No. 4,829,529 (1987), U.S. Pat. No. 4,815,079 (1989 Polaroid). The first cladding layer functions both as a cladding for the core waveguide and as the core of a larger multimode waveguide concentric with the core which guides pump radiation. The second cladding serves as a cladding for the first cladding waveguide. Rare earth doped multi-clad fibres having more than two cladding layers with successively decreasing refractive index are also known, for example US2002/0191928-A1 (June 2001). These are sometimes known as triple-clad, quadruple-clad etcetera. Multi-clad optical fibres are also known for other applications such as high power laser beam delivery U.S. Pat. No. 4,974,932 (1990), GB2379279 A (2001), or single-mode optical fibres with special dispersion properties used in optical telecommunications JP2001051147 (2001) or attenuation properties EP0783117 A2 (1997).
A recent development in multi-clad optical fibre design is known as large mode area fibre (LMA). An early example of such a fibre was demonstrated by Taverner et al, “158-μJ pulses from a single-transverse-mode large-mode-area erbium-doped fibre amplifier”, Optics Letters, 1997, Vol. 22, No. 6, pp. 378-380. The principal features of large mode area fibres are increased core diameter and reduced core numerical aperture compared to standard single-mode fibres. One advantage of such fibres is higher power handling due to the larger core and correspondingly smaller power density. Such fibres have become commercially available in the last five years or so and current embodiments are sometimes truly single mode and sometimes support multiple modes in the core. As an example, one commercially available fibre has a core diameter of 20 microns and a core numerical aperture of less than 0.07. It is sometimes desirable to construct a laser out of multi-mode LMA fibre that emits light only in the fundamental mode, see for example “Efficient laser operation with nearly diffraction-limited output from a diode-pumped heavily Nd-doped multimode fibre”, Optics Letters, 1996, Vol. 21, No. 4, pp. 266-268. A known technique for suppressing laser oscillation in the high order modes is to coil the fibre at a specific bend radius as demonstrated by Zawischa et. al. “All-solid-state neodymium-based single-frequency master-oscillator fiber power-amplifier system emitting 5.5 W of radiation at 1064 nm”, Optics Letters, 1999, Vol. 24, No. 7, pp. 469-471.
A drawback of some previous fibre laser designs is the susceptibility of the semiconductor pump lasers to damage caused by radiation that enters the pump laser diode cavity. This problem is exacerbated when the laser operates in a pulsed mode, and the peak output power of the laser is much higher than the average. A mechanism for pump diode damage is feedback of stray signal radiation back to the pump diodes; a relatively small fraction of such stray radiation may cause spurious diode outputs or damage the diodes. Stray signal light may occur at splice points and due to macro-bending loss if one or more of the fibres is coiled. Macro-bending loss is typically more significant in LMA fibres due to their increased bend sensitivity.
There is a need to improve the reliability of high power, diode pumped lasers and Amplifiers, and in particular for use with fiber lasers.